1. Field of the Invention
The present invention is within the field of electrographical printing devices. More specifically, the invention relates to an improvement to position a control electrode array in cooperation with a particle source to enhance the printing quality of direct electrostatic printers.
2. Description of the Related Art
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect printing process since the images are first formed on an intermediate photoreceptor and then transferred to paper surface.
Another form of electrostatic printing is known as direct electrostatic printing (DEP). Many of the methods used in DEP, such as particle charging, particle transport, and particle fusing are similar to those used in xerography. However, DEP differs from xerography in that an electric field is generated by electrical signals to cause toner particles to be deposited directly onto plain paper to form visible images without the need for those signals to be intermediately converted to another form of energy. The novel feature of the DEP concept is the simultaneous field imaging and toner transport to produce visible images directly onto plain paper or any suitable image receiving medium.
U.S. Pat. No. 5,036,341 granted to Larson discloses a DEP printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. The Larson patent discloses a method which positions a control electrode array between a back electrode and a rotating particle carrier. An image receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
An electrostatic field on the back electrode attracts the toner particles from the surface of the toner carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected individual control electrodes to create electrostatic fields which either permit or restrict the transport of toner particles from the particle carrier through the control electrode array. In effect, these electrostatic fields "open" or "close" selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged toner particles allowed to pass through the opened apertures impinges upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.
The control electrode array of the above-mentioned patent may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or a screen-shaped, apertured printed circuit. Generally, the array is formed of a thin substrate of electrically insulating material provided with a plurality of apertures. Each aperture is surrounded by an individually addressable control electrode, and a corresponding voltage source is connected thereto to attract the charged toner particles from the particle carrier to the image receiving substrate by applying voltage signals in accordance with the image information. For example, the control electrode array may be constructed of a flexible, non-rigid material and overlaid with a printed circuit such that apertures in the material are arranged in several rows and surrounded by electrodes. Regardless of the design or the material of construction, it is essential to maintain a constant, uniform gap distance between the control electrode array and the toner layer on the surface of the particle carrier.
The actual gap between the toner layer and the control electrode array can vary from machine to machine because the gap is determined by a combination of independent factors such as manufacturing variations in the size and placement of the particle carrier and the control electrode array, as well as the thickness of the toner layer on the particle carrier.
In addition to minimizing distance variations in the gap between the control electrode array and the particle carrier, it is also important to maintain a smooth uniform toner layer thickness on the particle carrier. Typically, the diameter of an individual toner particle is on the order of 10 microns, with a toner layer on the particle carrier being approximately 30-40 microns thick.
U.S. Pat. No. 5,666,147, also granted to Larson, discloses improved means for maintaining a constant minimal gap between the control electrode array and the particle carrier, while providing a uniform toner layer on the surface of the particle carrier. According to that patent, a spacer is mounted on the array on the side facing the particle carrier to engage the carrier on it, and the portion of the array supporting the spacer can move slightly radially towards and away from the carrier to accommodate imperfections in the carrier surface and variations in the toner layer thickness. The gap distance is thus maintained at a constant value according to the thickness of the spacer, independent of the thickness of the particle layer.
Further, to ensure entire coverage of the print area, the apertures are preferably aligned in several parallel rows arranged in a slight angle to each other, such that each aperture corresponds to a specific addressable area on the information carrier. Since the control electrodes are disposed around the apertures, the release area, i.e., the surface of the toner layer influenced by an individual control electrode, is larger than the aperture diameter. Since toner particles are supplied consecutively to the different rows, an overlap between the release areas of two adjacent rows will cause the toner supply to decrease from row to row. This defect, known as "toner starvation," causes a degradation of the print uniformity due to density variations between dots printed through different rows. Such a defect may cause print surfaces intended to be homogeneously covered by pigment to appear to be striped periodically in the direction of paper motion (white line noise).
In order to further reduce undesired distance variations of the gap between the control electrode array and the particle carrier, while providing a uniform toner supply to several aperture rows successively, there is still a need for improved means for positioning the control electrode array in relation to the particle source.